1. Field of the Invention
The present invention relates to an image forming apparatus using an amorphous silicon (hereinafter represented as "a-Si") photosensitive member utilizing electrophotographic system, and more specifically to an image forming apparatus including an electrophotographic apparatus employing a low cost photosensitive member comprising a cylinder with a small thickness.
2. Related Background Art
The photoconductive material constituting the photoconductive layer in the image forming member for electrophotography is required, in the image formation field, to satisfy various characteristics such as a high sensitivity, a high S/N ratio (photocurrent (Ip)/(Id)), spectral absorption characteristics matching the spectral characteristics of the irradiating electromagnetic wave (light in wide sense including ultraviolet light, visible light, infrared light, X-ray, gamma-ray, etc.), a high-speed light response, a desired dark resistance and no pollution to the human body at use. The above-mentioned pollution-free property at use is particularly important in the case of the electrophotographic image forming member incorporated in the electrophotographic apparatus to be used as an office equipment in the office.
Based on the above standpoints, amorphous silicon of which dangling bonds are bonded with monovalent elements such as hydrogen atoms (H) or halogen atoms (X) (hereinafter represented as "a-Si(H, X)") is described as to application to the electrophotographic image forming member, for example in the German Patent Application Laid-open Nos. 2746967 and 2855718, and is already utilized in the electrophotographic image forming member because it has excellent photoconductivity, abrasion resistance and heat resistance, and it is relatively easily formed with a large area.
In the case of forming an electrophotographic photosensitive drum with a photoconductive material containing a-Si(H, X), in order to obtain satisfactory photoconductive characteristics, there is generally employed a method of continuously heating a drum-shaped metal substrate, in an a-Si(H, X) film deposition apparatus, at a temperature of 200.degree. C. to 350.degree. C. which is extremely higher than that in the case of a selenium-based drum, and depositing an a-Si(H, X) film with a thickness of 1 to 100 .mu.m on the drum-shaped metal substrate. Such continued heating of the substrate at the high temperature is essential for obtaining the a-Si based photosensitive drum with excellent electrophotographic characteristics, and currently requires several hours to ten and several hours in consideration of the deposition rate of the a-Si(H, X) film.
In a preferred embodiment, the electrophotographic photoconductive member is composed of a drum-shaped or cylindrical metal substrate composed of Al or an Al alloy (hereinafter referred to as Al-based substrate) constituting the metal support member for the electrophotographic photoconductive member, and a photoconductive layer formed on the drum-shaped Al-based metal substrate and having an amorphous material containing silicon atoms as a matrix and preferably at least one kind of hydrogen atoms and halogen atoms. The photoconductive layer may also be provided with a barrier layer in contact with the drum-shaped metal substrate and further a surface barrier layer on the surface of the photoconductive layer.
FIGS. 1A and 1B are views showing an example of the structure of the a-Si photosensitive member. FIG. 1A is a schematic perspective view of the photosensitive member, wherein reference numeral 2100 indicates the thickness of the photosensitive member including a substrate 2101 and a light-receiving layer 2103. FIG. 1B is a schematic cross-sectional view of the photosensitive member. The photosensitive member includes a conductive substrate 2101 and a series of deposited sublayers constituting a light receiving layer 2105. On a conductive substrate 2101 such as of aluminum, there are successively deposited a charge injection inhibition layer 2102 for inhibiting charge injection from the conductive substrate 2101, and a photoconductive layer 2103 for generating electrons and positive holes by light irradiation and converting image information into potential information. Each of these layers is composed of amorphous silicon as a matrix and, if necessary, further contains a dangling bond neutralizing agent such as halogen atoms or hydrogen atoms, a valence electron controlling agent such as an element of the group III or V of the periodic table, a modifying material such as oxygen, carbon or nitrogen atoms. On the upper surface of the photoconductive layer 2103 as shown in FIG. 1B, there is provided a surface protective layer 2104 for protecting the photoconductive layer from the abrasion by a developer, a transfer paper and a cleaning device and for inhibiting the charge injection from the surface into the photoconductive layer. The surface protective layer 2104 is composed of a-SiC:H which is excellent in light transmission to the photoconductive layer, mechanical strength and prevention of charge injection from above.
The material constituting the drum-shaped metal substrate is preferably composed of, for example a metal such as NiCr, stainless steel, Al, Cr, Mo, Au, Nb, Ta, V, Ti, Pt or Pd, or an alloy thereof, and particularly Al or an Al-based alloy is preferably used.
As the material of the drum-shaped substrate, aluminum or aluminum-based alloy is preferred because satisfactory dimensional precision, for example in circularity or surface smoothness can be obtained relatively easily, also because the temperature control is relatively easy in the surface portion of the deposition of a-Si(H, X) at the manufacturing process and furthermore because such material is economical.
The halogen atoms (X) that can be contained in the photoconductive layer of the photoconductive member include fluorine, chlorine, bromine and iodine, but preferred is chlorine and particularly fluorine. In addition to the silicon, hydrogen and halogen atoms, the photoconductive layer may further contain, as the aforementioned valence electron controlling material or modifying material, a component for regulating the Fermi level or the bandgap such as atoms belonging to the group III of the periodic table such as boron or gallium atoms (hereinafter referred to as "group III atoms"), atoms belonging to the group V of the periodic table such as nitrogen, phosphor or arsine atoms (hereinafter referred to as "group V atoms"), oxygen atoms, carbon atoms, germanium atoms, alone or in suitable combination.
The barrier layer is provided for improving the adhesion between the photoconductive layer and the drum-shaped metal substrate or adjusting the charge receiving ability. The barrier layer is constructed with a single- or multi-layered structure composed of an a-Si(H, X) layer or a polycrystalline Si layer containing the group III atoms, group V atoms, oxygen atoms, carbon atoms, germanium atoms etc. according to the purpose.
On the photoconductive layer, there may be provided a surface charge injection inhibition layer or a protective layer consisting of an amorphous material containing silicon atoms as a matrix and carbon, nitrogen, oxygen atoms, etc. preferably in a large amount and, if necessary, further containing hydrogen or halogen atoms, or consisting of an organic substance with a high electric resistance.
The photoconductive layer composed of a-Si(H, X) can be formed with conventional various vacuum deposition methods utilizing electric discharge phenomena such as a glow discharge method, a sputtering method or an ion plating method.
In the following there will be explained an example of the method of producing the electrophotographic photoconductive member (photosensitive member) by the glow discharge decomposition method.
FIG. 2 shows an example of the apparatus for producing the electrophotographic photosensitive member by the glow discharge deposition method. A deposition chamber 1 is composed of a base plate 2, a wall 3 and a top plate 4. In the deposition chamber 1, there is provided a cylindrical cathode electrode 5, and a drum-shaped metal substrate 6 on which the a-Si(H, X) film is to be deposited. The substrate 6 is positioned at the central portion (concentric center) of the cathode electrode 5 and also functions as an anode electrode.
In order to form the deposited a-Si(H, X) film on the drum-shaped metal substrate in the above apparatus, a raw material gas introduction valve 7 and a leak valve 8 are at first closed, and a discharge valve 9 is opened to evacuate the interior of the deposition chamber 1. When a vacuum gauge 10 indicates about 5.times.10.sup.-6 Torr, the raw material gas introduction valve 7 is opened to introduce, into the deposition chamber 1, mixed raw material gases such as SiH.sub.4, Si.sub.2 H.sub.6, SiF.sub.4, etc. adjusted at a previously predetermined ratio in a mass flow controller 11. The opening degree of the discharge valve 9 is adjusted under the observation of the reading of the vacuum gauge 10, so as to maintain the pressure in the deposition chamber 1 at a predetermined value. Then, after confirmation that the surface temperature of the drum-shaped metal substrate 6 is set at a predetermined value by a heater 12, a high frequency power source 13 is activated at a desired power to generate glow discharge in the deposition chamber 1.
During the layer formation, the drum-shaped metal substrate 6 is rotated at a constant speed by a motor (M) 14, in order to achieve uniform layer formation. Thus an a-Si(H, X) deposition film can be formed on the drum-shaped metal substrate 6.
However, the deposited a-Si(H, X) film often peels off from the drum-shaped metal substrate not only during the film deposition in which the drum-shaped metal substrate is maintained at a high temperature but also during the cooling to the atmospheric temperature after the film deposition, because of a difference in the thermal expansion coefficient between the drum-shaped metal substrate and the a-Si(H, X) film and also because of a large internal stress in the formed a-Si(H, X) film. Besides, the peeling of the a-Si(H, X) film is often observed in the course of use as the electrophotographic photosensitive drum, by the heating of the drum depending on the ambient temperature in the use. Such peeling of the a-Si(H, X) film tends to occur more easily with an increase in the thickness thereof, and is also caused in the case of the a-Si(H, X)-based photosensitive drum. By a thermal deformation of the drum-shaped metal substrate (in particular, easily during the formation of the photoconductive layer) at a level not inducing the film peeling in the conventional Se-based photosensitive drum, the film peeling often generates in the a-Si(H, X)-based photosensitive drum because of the difference in the thermal expansion coefficient as mentioned above and the large internal stress of the a-Si(H, X) film. The internal stress of the a-Si(H, X) film can be relaxed to a certain extent by the producing conditions of the a-Si(H, X) film (such as the kind of the raw material gasses, gas flow rate ratio, discharge power, substrate temperature, internal structure of the producing apparatus, etc.), but such relaxation is still insufficient in consideration of the productivity and mass production efficiency. The film peeling is fatal, inducing an image defect in the use of the electrophotographic photosensitive drum.
Also the heating of the drum-shaped metal substrate at the high temperature for a long time in the production of the a-Si(H, X) film not only induces the film peeling as mentioned above but also tends to cause thermal deformation of the drum-shaped metal substrate. This thermal deformation causes uneven discharge in the production of the a-Si(H, X) film, thereby degrading the uniformity of the deposited film and resulting in an image defect.
In consideration of the foregoing, there is already proposed, as disclosed for example in the Japanese Patent Publication No. 6-14189, an electrophotographic photoconductive member capable of reducing the image defect by employing a drum-shaped metal substrate composed of aluminum or an aluminum alloy having a thickness of at least 2.5 mm.
However, in consideration of the recent fierce price competition in the copying machine market spreading particularly in the middle- and low-speed machine area, a lower running cost alone is insufficient and the realization of a lower initial cost is an important point. For this reason, it is urgently desired to reduce the cost of the substrate and significantly reduce the cost of the photoconductive member.
Within the cost of the photoconductive member, the raw material cost has a large proportion, and a reduction in the thickness of the drum-shaped metal substrate is anticipated not only to simply reduce the raw material cost but also, because of a low heat capacity resulting from the smaller thickness, to achieve various cost reduction effects such as electric power saving and a shorter tact time based on a shorter heating time, a reduction in the electric power required for maintaining the high temperature, and a reduction in the tact time based on a reduced cooling time in the production of the a-Si(H, X) film. For these reasons, there have been desired a lower cost of the drum-shaped metal substrate and an improvement in the temperature characteristics thereof.